First a 100 GHz Dense Wavelength Division Multiplexing (DWDM) at the central wavelength of 1550.12 nm is inserted to filter out the crosstalk noise from running fiber links of classical communication which are in the same bundle with the QKD link, and to filter out the inelastic scattering noise caused by strong reference pulses [1]}. In the following a circulator (CIR) is used to block the noise caused by the reflection of the four superconducting nanowire single-photon detectors (SNSPDs) [1]}, after that the pulses are polarized and split by the electric polarization controllers (EPCs) and polarization beam splitters (PBSs), the idle beam of the PBS is detected to carry out the arrival time and polarization feedback, another beam is sent into a polarization-maintaining beam splitter (BS) to accomplish the single photon interference. All the detections are recorded by a high speed multichannel time tagger.
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